The words "easy" and "megaproject" do not belong in the same sentence, much less consecutively, unless preceded by "not."
Think up the most impractical thing that you've ever thought of adding to your fortress, and then build it. Out of soap.
That's not fair, casting is pretty easy, just boring.
Hmm. Depends on one's definition of easy. Casting is definitely fairly simple; build scaffolding, fill with magma/water, remove scaffolding and shape to taste. It gets difficult to do if you want to avoid casualties, or if you want to cast it to shape. OTOH, building a giant steel pyramid out of the melted armor of your adversaries requires minimal effort and dwarfs, but is not necessarily simple, especially the first time or if you want to cap it with masterwork statuary like I do.
If you want an easy megaproject, and have magma and infinite water/sand available, try building a pyramid to house the heroes of your fort, out of either glass or obsidian. Setting and cranking up a glass industry is a macroproject of its own, as is a fair-sized obsidian farm. Next fortress I make I'll have to see if I can get a decent 20x20 obsidian farm going. After that, each layer is easy enough; a square of walls, followed by a square of ramps around those walls.
Side and top view
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